A protective layer approach to solvatochromic sensors

Abstract

As they have been designed to undergo colorimetric changes that are dependent on the polarity of solvents, the majority of conventional solvatochromic molecule based sensor systems inevitably display broad overlaps in their absorption and emission bands. As a result, colorimetric differentiation of solvents of similar polarity has been extremely difficult. Here we present a tailor-made colorimetric and fluorescence turn-on type solvatochromic sensor that enables facile identification of a specific solvent. The sensor system displays a colorimetric transition only when a thin protective layer, which protects the solvatochromic materials, is destroyed or disrupted by a specific solvent. The versatility of the strategy is demonstrated by designing a sensor that differentiates chloroform and dichloromethane colorimetrically and one that performs sequence selective colorimetric sensing. In addition, the approach is employed to construct a solvatochromic molecular AND logic gate. The new strategy could open new avenues for the development of novel solvatochromic sensors.

Figure 1. Fabrication of the solvatochromic sensor system.

(a) Schematic representation of the colorimetric sensor system. (b) Fabrication of polyacrylic acid (PAA)-protected polydiacetylene (PDA) film on a glass substrate.

Figure 2. THF-specific solvatochromism.

(a) Photographs of solvent-exposed unprotected (top) and PAA-protected (middle) PCDA-derived PDA films. Fluorescence microscope images of the solvent-treated PAA-protected PDA film are also displayed (bottom). Photographs were taken 1 min after adding a drop (ca. 100 μl) of each solvent to the top of the film. (b) Absorption spectra of PAA-protected PDA film upon exposure to various solvents. (c) Film thickness as a function of distance before (blue line) and after (black line) PAA protection. The fluctuating red line is obtained after exposure of the PAA-protected PDA film to THF. (d) Plots of PDA chromic power versus PAA penetrating power for various solvents tested. (e) Plots of colorimetric response time and film thickness as a function of concentration of protective PAA.

Figure 3. Colorimetric differentiation between dichloromethane and chloroform.

(a) Schematic of a double-layer protected sensor system. (b) Photographs of solvent-exposed single- (top, PVC), and double- (bottom, PVC-on-PVP) layer protected PCDA-derived PDA films. Photographs were taken 1 min after adding a drop (ca. 100 μl) of each solvent to the top of the film. (c) Absorption spectra of double-layer protected PDA films upon exposure to chloroform and dichloromethane. (d) Plots of PDA chromic power versus layer penetrating power for chloroform and dichloromethane.

Figure 4. IPA selective solvatochromic sensor.

(a–c) Photographs of the solvent-exposed unprotected (a), single-layer protected (PAA/PDA) (b) and double-layer protected (PVP/PAA/PDA) (c) TCDA-derived PDA films. Photographs were taken 1 min after dropping each solvent on the top of the film.

Figure 5. Sequence selective logic gates.

(a) Schematic of a double-layer protected PCDA-derived PDA film for a sequence-specific colorimetric sensor. (b) Photographs of the sensor films after exposure to IPA and toluene in different order. (c) A solvatochromic keypad lock logic circuit. (d) Film thickness as a function of distance before (black) and after protection with PMMA (blue) and PVP (green). The red line is obtained after exposure of the sensor film to IPA.

Figure 6. Solvatochromic AND logic gates.

(a) Schematic of a double-layer protected PCDA-derived PDA film for an AND logic gate. (b) Photographs of the sensor film after exposure to H₂O, THF and 20% aqueous THF. (c) A two-input solvatochromic AND logic gate. (d) Colorimetric response of the sensor film after exposure to various volume compositions of water in THF.

Figure 7. A fluorescence turn-on solvatochromic sensor system.

(a) Schematic representation of a fluorogenic sensor system based on fluorescamine-amine adduct formation. (b) Photographs of vials containing solutions of fluorescamine (2 mg ml⁻¹) and amine (2 mg ml⁻¹) under 365 nm UV light. (c) Photographs of unprotected films (without PVP layer) under 365 nm UV light. (d) Photographs of PVP-protected films under 365 nm UV light. (e) Photographs of PVP-protected films under 365 nm UV light after exposure to various solvents.